The present invention generally relates to a light valve projector of the Schlieren dark field type and more particularly to an apparatus utilizing two light valves one of which has an optical pupil comprising an input mask having an improved light-transmissive slot configuration which in conjunction with the corresponding output mask fully utilizes the slots which are available in the input and output masks.
The general principles and mechanisms of a light valve projector utilizing Schlieren projection lenses are described for example in U.S. Pat. No. 3,437,746 issued to Good et al., on Apr. 8, 1969 which is incorporated by reference herein for the purpose of disclosure. Such a system utilizes a single light valve to project color images using a common area of a viscous light modulating medium, or control layer, and a common electron beam for the production of deformations in the control layer for simultaneously controlling the transmission therethrough, point by point, of the primary color components in a beam of light in response to a plurality of simultaneously occurring electrical signals.
In a light valve projector system the input bar plate, or mask, passes filtered light focused onto the slots of the input mask by a lenticular lens system composed of arrays of lenslets. The output bar plate or mask blocks or passes light rays according to whether they have been diffracted by diffraction gratings written on the control layer by the electron beam. The configuration of the input and output masks is complementary; that is, opaque areas of the output mask are aligned with slot (i.e., transparent areas) of the input mask while slots of the output mask are aligned with opaque areas of the input mask. The configuration of the input mask dictates the configuration of the color filter plate and the lenticular lens system.
A single light valve full color projector has a typical modulated light output of about 1200 lumens and is acceptable for many applications. However, there are some applications such as simulator displays, arena displays, and for large audience screens over 20 foot wide where higher light output and resolution are required. Multiple light valve (MLV) projectors utilizing two light valves, one to produce green light and a second, to produce red and blue light (magenta) are used for such applications. Such MLV projectors are capable of achieving a 2000 lumen light output but not with a consistent, comfortable margin. Variations in light source, dichroic color and efficiency, light valve efficiency, and the scanning rate determine whether or not an output of 2000 lumens is achieved. An MLV projector is described in U.S. Pat. No. 3,265,811 issued to Ellis on Aug. 9, 1966 which is incorporated by reference herein for disclosure purpose.
A two color, blue and red, light valve is shown in FIG. 1. Briefly, with reference to FIG. 1, the slots of the input mask 20 are arranged such that the horizontally oriented slots, referred to as "red slots", are in the center of the Plate and the vertically oriented slots, referred to as "blue slots", are located above and below the red slots. As mentioned, the output mask 48 is complementary to the input mask; i.e., the opaque bars in the output mask are aligned with the slots in the input mask 20. An enlarged view of the prior art input mask 20 is shown in FIG. 2 where, for purposes of illustration, a portion of the input mask 20 is shown broken away to reveal arrays of lenslets or lenticules 19 of the lenticular lens system 16. 18. In this type of input mask, a horizontal slot 22 is aligned with every row of lenticules and the adjacent slots 22 are separated by an opaque horizontal bar 24.
The configuration shown in FIG. 2, having five horizontal slots 22 occupying a first region or central portion of the input mask 20, and seven vertical slots 30, separated by opaque bars 32, located in a second region comprising portions above and below the central portion, is a compromise design for highest possible resolution rather than highest light output. The horizontal slots 22 and the vertical slot 30 are arranged on a one-to-one basis with the vertices of the lenticules 19 and have a geometric transmission of 25%; i.e., the input slots occupy 25% of the center-to-center spacing of the lenticules. It has been determined that for a 525 line scanning rate, the peak instantaneous modulated light efficiency for the horizontal slots 22 is 28.43% (including first, second and third order ray utilization), but for a 1023 line scanning standard, the peak efficiency decreases to 22.86% because the angle of diffraction corresponding to the second order ray utilization also corresponds to the angular spacing of the output bars (or input slots because they have a one-to-one relationship) which results in complete blockage of the second order rays.
A multiple light valve (MLV) projector apparatus for producing a color display includes two separate light valves of the Schlieren dark field type. One of the light valves produces a first primary color, e.g. green, and the other light valve produces two colors, e.g., red and blue. Referring again to FIGS. a and 2, there is shown a typical two color light valve projector apparatus which may incorporate the present invention. The apparatus includes, in sequence, a lamp 10 which provides white light to an elliptical reflector 12, a color filter plate 14, a first lenticular lens plate 16 having located on the surface remote from the lamp an array of first lenticules, or small lenses, 17 and an input pupil comprising a second lenticular lens plate 18, having located on the surface adjacent the lamp an array of second lenticules 19 and having located on the opposite surface an input mask 20. The prior input mask 20, shown in FIG. 2, includes a first region, or central portion, having a plurality of horizontally extending first slots 22 separated by opaque bats 24. A centrally disposed aperture 26 is provided therein for an electron gun 28, schematically shown by the dashed lines. A second region of the input mask 20 includes portions above and below the central portion. A plurality of vertically extending second slots 30 are formed in the second region and are separated by opaque bats 32. A section of the input mask 20 has been broken away to show that the second lenticules 19 of the second lenticular lens plate 18 are arranged in horizontally extending rows and vertically extending columns to form an array of lenslets. Five horizontal slots 22 are formed in the central portion and seven vertical slots 30 are formed the portions above and below the central portion of the input mask 20. Both the horizontal slots 22 and the vertical slots 30 are arranged on a one-to-one basis with the vertices of the second lenticules 19. The slots 22 and 30 have a geometric transparency, or width, of 25% of the center-to-center spacing of the second lenticules 19. However, since the second lenticules 19 are substantially rectangular and have a 3-to-4 aspect ratio, the horizontal slots 22 are closer together and narrower than the vertical slots 30 by the 3-to-4 ratio.
The second lenticular lens plate 18 forms a portion of the envelope of a sealed light valve 34 which includes therein the electron gun 28, the input mask 20, deflection plates 36, a rotating disk 38, and a fluid reservoir 40. A control oil layer is provided on the rotating disk 38 as it rotates through the sump or reservoir 40 and information is written on a control layer raster plane 42 by an electron beam (not shown) from the electron gun 28. A Schlieren-projection lens 44, including a Schlieren lens 46, the output color-selection mask 48 and a projection lens 50, is secured to the end of the light valve 34 opposite the electron gun 28. The output mask 48 is the complement of the input mask 20. That is, in the output mask 48 the horizontal and vertical slots (shown schematically in FIG. 1) are aligned with the horizontal and vertical opaque bars 24 and 32, respectively, of the input mask 20, and the horizontal and vertical slots of the output mask 48 are aligned with the horizontal and vertical slots 22 and 30, respectively, of the input mask. Of course, there is no central aperture in the output mask 48. Thus, if the control layer is undisturbed, i.e., no information is written thereon, the bars of the output mask 48 block the light transmitted through the slots 22 and 30 of the input mask 20 and there is no light output from the light valve projector. This is a basic characteristic of a light valve projector of the Schlieren dark field type.
The embodiment of the two-color light valve projector shown in FIGS. 1 and 2 uses orthogonal diffraction axes and appropriate spatial filtering to achieve independent control of pixel color and intensity. The input pupil of the light valve 34 is split up chromatically and by spatial filtering such that red light chromatically and by spatial filtering such that red light from the filter plate 14 passes through the horizontal slots 22 and blue light from the filter plate 14 passes through the vertical slots 30. A high frequency carrier, e.g., 48 and 96 MHz at a 525 line scan rate, and 96 MHz only, at a 1023 line scan rate, is applied to the vertical elements of the deflection plates 36 and is modulated by the red video signal. The electron beam writes a modulated raster or first diffraction grating on the control layer raster plane 42. A second diffraction grating is created orthogonal to the first grating by velocity modulating the electron beam in the horizontal direction by applying a harmonically related carrier signal, e.g., a subharmonic such as 12 MHz at the 525 line scan rate and 24 MHz at the 1023 line scan rate, to the horizontal plates and modulating it with the blue video signal. The first diffraction grating on the control layer raster plane 42 diffracts the red light vertically so that it passes through the complimentary horizontal slots formed in the output mask 48 while the second diffraction pattern on the control layer raster plane 42 diffracts the blue light horizontally through the complementary vertical slots in the output mask.